Currently nonhuman primates are the only genus that is susceptible to simian immunodeficiency virus (SIV) and develops a disease nearly identical to that seen in humans infected with HIV. In humans, the individual response to HIV infection is variable and patients demonstrate marked variability in chronologic progression to clinical AIDS. The time interval between infection and clinical disease is highly variable and depends on both viral and host factors. A small percentage of infected patients apparently are able to resist disease progression. Research into viral/host factors may provide information of clinical and therapeutic importance, and these long-term survivors may be an important group to study. We currently have four pigtailed macaques that have been infected with SIV/HIV for many years and fall into the category of long-term survivors. None of these animals has yet progressed to clinical AIDS. These animals are important to maintain for future research and provide an important animal model for disease progression and pathogenesis in individuals surviving infection for long periods of time. One macaque was inoculated with SIVmne E11S in 1988 and has remained seronegative although virus was isolated at 174 and 195 weeks post-inoculation. This animal has developed a significant decline in CD4+ cells but has yet to develop disease. In addition, three macaques were inoculated with HIV-1 in 1992 and virus has not been isolated since 10 weeks post-inoculation. Two of the animals remain strongly seropositive, which provides strong evidence for continued viral replication. To date these animals have not progressed to clinical disease.